Allow proxy minions to load static grains
Allow proxy minions to load static grains

Add the `__proxyenabled__` global var so the extra grains are loaded.
Inside the `config` function of the extra grains check if the minion
is a proxy, then try loading from <conf_file>/proxy.d/<proxy id>/grains.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt